High density polyethylene (HDPE, density from 0.941 g/cm3 to 0.965 g/cm3) is commonly used for injection molding applications. HDPE resins have excellent tensile strength, abrasion resistance, stress crack resistance, and chemical resistance. They are used for making crates, boxes, refuse carts, and waste bins. One problem associated with injection molding of HDPE is that HDPE resins often exhibit high warpage in molded articles, particularly when they are used with certain colorants or pigments. Warpage is caused by uneven shrinkage in the machine and transverse directions.
Many new pigments have been developed for HDPE coloration. See, for example, U.S. Pat. Nos. 6,555,604 and 7,153,358. These new pigments may offer reduced warpage to molded articles from the colored HDPE. However, they are more expensive than the traditional pigments such as phthalocyanine blue. New colored HDPE compositions are needed. Ideally the new colored HDPE composition uses traditional pigments and has minimum warpage in injection or other molding applications.